Setting Loop Flowrate for Efficiency

Discussion in 'Maintenance and Troubleshooting' started by Caveman, Dec 25, 2010.

  1. Caveman

    Caveman New Member

    Help Please! I’ve purchased a Geo system that I have to learn to tweek because the service people performing the routine checks clearly don’t know what they’re doing. (the last guy tried to take pressure readings w/ a gauge that had increments of 5psi). I’m not seeing hardly any decrease in total power bill despite going from a 15-20 year old 80% efficiency gas furnace/AC. My new system is a NDV038A111CTR w/ hot water generator, preheat tank and a closed 350 ft SDR11 polyethylene plastic loop (3/4 inch outside the house, 1.25 inch inside) filled w/ 21% methanol in a horizontal loopfield with the loop spaced 10 feet apart, and I’m located in Indianapolis w/ somewhat sandy soil since I’m not far from a river.

    In reading this forum and there seems to be some debate about what is the optimal flowrate for efficiency, some thinking the faster the better, some saying stay within the range on the installation manual(2.5-3.0GPM/ton… in the case of my unit 7.9-9.5gpm), and the manufacturer saying set it at 9.0GPM (but I’m guessing this is a balance between efficiency and unit safety, longevity). My loop installer seems to be the most knowledge person among the Heating and Cooling company’s service techs and the Waterfurnace tech service department… and he thought 9.0gpm was ideal in the summer, but a slower rate is better in the winter….thinking that a fast flowrate in the summer allows heat to be taken out of the house faster, but a slower flowrate allows more heat to be picked up from the loopfield in the winter.

    1) Does anyone have more thoughts on what is the optimal flowrate? If not, do I just find it experimentally by finding the greatest temperature difference in return and supply via testing different loop flows?
    2) If a slower flow rate does turn out to be more efficient, I believe I heard that there are certain problems that I need to look out for (faults,etc). Can anyone elaborate on this (and would the warning signs, issue be different in summer than the winter?)

    I guess I’m going to be making some purchases in order to be able to test flowrate. Will probably buy the following:
    [*]a cheap clamp-around amp meter to make sure I'm in Stage 2 when testing
    [*]A SISCO GA-125 gauge adapter
    [*]Either a 100 psi gauge with 1 psi increments or a GPDW-08 digital gauge (0.1psi increments)
    [*]A snubber if pressure fluctuation is too much to get a good estimation.
    [*]Temperature probe(s) with 0.1F increments for water readings and return/supply air readings.

    3) On the temperature probe, does anyone have any recommendations?

    Any advice is appreciated.
  2. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    I am not sure why your power bill is high, unless your strip heat is running often,

    I am sure that the high power bill is not related to the loop flow. The difference in efficiency from 7.9 to 9.5 gpm is almost too small to calculate.

    The first thing you need to know and pass on to us is the EWT. This is the temp of the water as it enters the heat pump from the loop. Also the LWT.

    Do a google search for :

    cooper dfp450w digital pocket thermometer.

    These seem to work the best.

    Good luck
  3. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I will add

    most manufacturers publish charts for their equipment showing the heat "pumping" that goes on at various flow rates and fluid temperatures. That is where I would start. The flow rate can be determined using pressure guages and I intall them in every system I build or work on. The latest and greatest way to measure water tempuratures at a reasonable cost are digital aquarium thermometers.

    Take some measurements and we will be able to offer opinions and courses of action.
  4. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Unless you have a variable speed loop pump (rare) you won't be "setting" the flow rate unless by throttling it using a valve. It generally makes no sense to throttle a loop pump using a valve unless the flow through the unit is high off the chart, in which case you hustle to order a smaller pump.

    The idea that heat transfer is enhanced by SLOWING flow is a peculiarly enduring myth that I shoot on sight...(locking and loading an example now):

    You climb into your car on a sweltering day. The car has AC, so you turn it on. Do you select the lowest fan speed so the air has more time to be cooled by the AC system and then more time to cool your body by moving more slowly...NO - you crank that puppy up! When you are approaching comfort you back off the fan speed a bit so you can hear the radio and yourself think.

    I'm with Dewayne - 8 vs 9.5 GPM through an 038 makes not a whit of difference...inquire elsewhere

    Air and waterside delta-Ts (inlet and outlet temperatures) will help us dig deeper. Also try to determine whether and for how long aux heat strips are coming on - they are huge power suckers.
  5. Bergy

    Bergy Member Industry Professional Forum Leader

    Mark, I know you do high quality work but I must disagree with you on this point. One can not rely on separate gauges to calculate the Heat of Extraction/Rejection. This must be done by a single temperature probe and a single pressure gauge, eliminating differences between differing probes.

  6. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    If checking the delta P

    through the machine and comparing it to the manufacture's specs does not get you a rough flow rate, why do they publish the tables?

    I ment the use of guage pressure differances to reveal flow as an economical alternative to high cost flow guages.
    Last edited: Dec 30, 2010
  7. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I don't think Bergy is arguing against using delta-P across heat exchanger to infer flow. Rather he is arguing that the absolute (in)accuracy between different pressure gauges is such that it injects enough uncertainty (just one psi would throw things far off) to render meaningless the delta-P calculation.

    Mark, I'll bet you get around this by using sufficiently high quality gauges...if they have a calibration screw it would be a simple matter to adjust them so they show no delta-P when the system is doesn't care about absolute accuracy, just that they follow each other.

    In industrial situations simple flow transmitters have two inputs, one from each side of an artificial restriction such as an orifice plate. The device reads delta-P across the two inputs and reports a flow from that.
  8. Looby

    Looby Member Forum Leader

    Very true. Also note that the higher flow rate will yield (slightly) better
    heat transfer both in the loop and coax -- in both summer and winter.
    So, reducing the flow by throttling is a guaranteed waste of effort and
    loss of efficiency.

  9. Looby

    Looby Member Forum Leader

    BTW, it's also possible to set a GPDW-08 to display pressure in FTWC
    (feet of water column). That doesn't improve accuracy, but it more than
    doubles the resolution of your pressure readings. That's especially
    useful with WF Envision, where delta-Ps are typically only 2.5 - 3.5 psi
    (or 6 - 8 FTWC).
  10. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    As much as Curt and I

    kid each other about test equipment, He is correct about the quality of guages I use. I learned to work with these systems before there where PT ports and flow centers. At a glance I can see my flow due to delta P. I am changing the type of hydronic thermometers I use as they do not quite go low enough for winter EWT.
  11. Caveman

    Caveman New Member

    EWT and LWT numbers

    Hi all
    I had the service techs back in today. we took readings at stage 1, stage 2 at the loop flowrate of the past 3 months, and stage 2 at maximum loop flowrate.

    stage 1
    EWT= 29.0
    LWT= 33.0
    pressure in= 63psi
    pressure out=58psi

    stage 2, moderate loop rate
    pressure in= 63.5psi
    pressure out=58psi

    stage 2, maximum loop flowrate
    pressure in= 65psi
    pressure out=59psi
  12. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    The numbers look OK

    but I think EWT and LWT numbers are backwards as I doubt you are in cooling.

    Without the charts for your machine I guess you have enough flow.

    I am falling inline with Dewayne in that it is time for the manufacturers to allow a consumer interface showing strip heaters are on.
  13. geome

    geome Member Forum Leader

    In a user sub-menu (Equipment Status) on our Prestige thermostats, I can see On or Off for 1st Stage, 2nd Stage, & Aux Heat (and other stuff like filter days left, etc.)

    I just tested our thermostat to see what the main display shows during various stages:
    1st stage shows "heat on"
    2nd stage shows "heat on"
    Aux shows "auxiliary heat on"
    Emergency heat shows "emergency heat on"

    While this is nice, people shouldn't have to spend what I did to easily see this information. Now if I can just get the smell of burning dust out of here...
  14. Looby

    Looby Member Forum Leader

    Caveman, what circulating pump(s) are you using?

    Your 5-6 psi delta-P readings are "off the charts" for the heat exchanger
    in a WF NDV038. At 9 GPM and 30°F, delta-P should be only 3.4 psi.

    Notice that increasing from "moderate" to "maximum" flow resulted in
    almost no change to delta-P or delta-T. This suggests that even at
    "moderate" flow you're already past the point of diminishing returns
    for efficiency (or capacity) vs. flow rate with that heat exchanger.

    Delta-P is highly nonlinear with flow rate, and once you hit the wall,
    "pushing harder" has little effect on GPM. It only wastes power.

    If you have more than one circ pump, it might be more than you need.

  15. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I am getting this right, you have only one loop of 0.75" pipe, 350' long? That appears to be a very short loop, especially in sandy soil. Is the soil dry or well saturated? Your temp seem to be in order, and your heat extraction is kind of high giving your pressure drop. So something is off. I simply assume your LWT your LWT.

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